Diabetes drug metformin linked to low levels of thyroid stimulating hormone

Use of the type 2 diabetes drug metformin has been linked with lower levels of thyroid-stimulating hormone (TSH).
Low levels of thyroid-stimulating hormone can indicate an increased risk of heart disease and bone fractures but with previous studies showing metformin to have a relatively good heart health safety record, patients taking the drug should not be unduly worried.
The news comes after a Canadian study found that in patients with type 2 diabetes and hypothyroidism (an underactive thyroid), the risk of having low levels of thyroid-stimulating hormone was 55% higher in patients that took metformin compared with those that took a sulphonylurea. Low levels of TSH were only observed within patients with hypothyroidism and not in those with normal thyroid levels.
Readers should not allow tabloid headlines to get them unnecessarily worried. The Daily Express has interpreted the story as ‘Common diabetes pill dramatically raises risk of heart disease’, and greatly overstates the risks beyond any findings of the Canadian study.
Whilst low levels of TSH have been linked with heart disease, the study did not look for or show any links between metformin and heart disease. By contrast, previous research studies that have specifically looked at cardiovascular risk have shown that metformin has a better health record in terms of heart risk than the other most commonly prescribed type 2 diabetes drugs such as sulphonylureas.
Whilst metformin has a good heart health record, the Canadian researchers note that further research is warranted to investigate whether there are any health risks, or not, associated with metformin use in patients with hypothyroidism.